A Method to Cut Biofuel Costs

Cellulosic ethanol, which is produced from the inedible fibers in plant and crop residues, fast-growing grasses and woodchips, has often been hailed as the holy grail of biofuels. It’s made from low-cost and plentiful feedstocks; it has the potential to be widely produced yet doesn’t pit food against fuel, as accused in corn-based ethanol production; and it has a smaller carbon footprint than either petroleum or ethanol from corn.

There’s just one catch: cost. While the feedstocks are cheap, the process of turning them into useful chemicals and fuels isn’t. With a high cost per gallon to convert the tough plant fibers into fermentable sugars, large-scale production has so far proved too pricey to be competitive in the marketplace. That outlook might soon change, however, thanks to a discovery by scientists at Ames Laboratory that could remove one of the biggest cost barriers to commercialization.

Javier Vela, an Ames chemist, has found a way to make photocatalyst nanoparticles that are more stable and require only sunlight to carry out the key fiber-to-sugar conversion— developments that could reduce costs of catalyst replacement while eliminating the need for expensive external energy inputs.

“The main way people covert biomass into fuels or chemicals is by thermolysis or heating,” said Vela, who is also an assistant professor of chemistry at Iowa State University. “It’s very energy-intensive, as you start by wasting a lot of energy just to break all these big plant cell molecules apart. If you can get rid of that heating step, you’re already making the process more efficient.”

With Vela’s new solar energy-harvesting photocatalyst, the heating step may no longer be needed. He’s discovered a way to use nanoparticles made from naturally strong light absorbers. He and graduate student Purnima Ruberu created a photocatalyst capable of absorbing even more light—importantly, in the desired red to near-infra-red spectrum for solar energy—and powering a chemical reaction based only on the sun’s energy. “For the most part, these sunlight photochemical reactions with biomass substrates have not really been explored,” Vela said. “But if you think about it, the best photocatalyst is actually nature—so, leaves and photosynthesis.”

In an effort to more closely mimic how plants create energy through photosynthesis, where chloroplasts in plant cells use sunlight to convert carbon dioxide into sugars that feed them, Vela is working with colleagues at Ames Lab on encapsulating these light-harvesting rods inside transparent porous supports.

These supports contain the rods while still allowing light infiltration and exchange with the outside medium. And because they are largely insoluble, they can be more easily collected and reused.

“The porous supports are the synthetic equivalent of a chloroplast,” Vela said. “By putting our photocatalyst in them, we hope to control how fast those reactions occur—so that instead of getting two or 10 percent energy conversion, we get 100 percent conversion.”

As with many important discoveries, Vela’s happened by accident. He originally wanted to create a rod-shaped photocatalyst made of just cadmium and selenium—elements that together absorb light strongly in the preferred region of the solar spectrum. But due to certain properties of those elements, the process of making rods frequently produced other shapes not as conducive for solar energy harvesting.

He had recently learned, however, about a simple and reproducible process of making rods from cadmium mixed with sulfur. While those elements have a bluer band gap, Vela surmised that following the procedure for making cadmium-sulfur nanorods, but adding small amounts of selenium, would achieve the desired shape, while skewing the final mixture into a redder band gap range.

He was right about the redder band gap, but the resulting shape—drumstick-like particles with thick “heads” and narrow “tails”—seemed to run counter to the laws of chemistry.

“Usually when you mix two or more semiconducting materials together, the optical properties tend to be an average of the pure materials,” Vela said. “The shape we got just didn’t make sense. According to solid-state chemistry, you should be able to make a solid solution of, in this case cadmium-sulfide and cadmium-selenide, where you have a homogenous distribution of sulfur and selenium across the rod structure. Instead what we got was a head region very rich in selenium and another area that’s very thin and rich in sulfur. We didn’t understand at first why there was that segregation of materials.”

Further investigation revealed a simple explanation: The sulfur and selenium precursors Vela used had mismatched reactivities, which caused them to form at different times resulting in the drumstick shape. While he didn’t unearth some new law of nature, stumbling on a way to segregate elements in a solid-state mixture proved key to discovering how to make the cadmium-sulfur-selenium photocatalyst more stable across multiple uses.

“In our photocatalyst design, we need a second particle—a metal island, or co-catalyst,” Vela said. “Once you form that electron-hole pair, you need to separate those charges, and it’s the metal island that does that. We never would have been able to figure out how to do that without having made those drumstick structures with two different poles.”

This metal island is what bestows enhanced longevity on Vela’s photocatalyst. The ones lacking the metal island completely dissolved after eight days, while he said that those with it remained active. Currently, the enhanced photocatalysts survive about two weeks, a considerable boost in recyclability. Extending that durability even more is merely a matter of optimizing, he said, not functionality.

“We’ve shown that the photocatalyst works—that the reaction really works, at room temperature, driven just by sunlight,” Vela said. “I’m sure it can be optimized.”

Currently, Vela and colleagues are working to perfect the method for encapsulating the rods within porous networks and he’s optimistic about where the work is heading, and what it could mean for the future of biofuels processing. Another promising aspect of his research is its transferability to other materials—a fact that would be particularly important if ever questions were raised about cadmium toxicity.